Our ability to smell thousands of different odors is mediated by a large repertoire of odorant receptors (OR) expressed on sensory neurons in the nose. The Ors bind odorous molecules and transmit this information to the brain. During development, Ors also guide neurons to spatially fixed relay stations in the olfactory bulb. Ors form one of the largest gene families in the mammalian genome, with at least 1000 members. OR genes are distributed on many chromosomes in clusters that may occupy as much as 1 percent of the genome. The family also includes many pseudogenes and OR- like genes that may perform other chemosensory functions, such as cell-cell recognition or sperm chemotaxis. This project s goal is to learn (A) how the diversity of the expressed repertoire of Ors has evolved to meet the differing demands for olfaction in mouse and human; (B) how transcription of these genes is controlled such that each sensory neuron expresses a single OR, while a diverse repertoire is displayed in the nose as a whole; (C) how the specificity of ligand-binding and neuronal-targeting is embodied in the structure of the OR proteins; (D) whether the complex genomic organization of Ors is based on functional considerations; (E) how the OR genes responsible for the sense of smell are distinguished from and organized relative to other members of the family; (F) how the evolutionary dynamic nature of the OR subgenome impacts the functional repertoire; and (G) how the repertoire varies among individuals or with age exposure to odors. We therefore propose to conduct a comprehensive and detailed genomic analysis of the OR families in mouse and human and to integrate these genomic data with assays of gene function and regulation. Aim 1 is to characterize nearly the full repertoire of Ors active in the olfactory tissue of mouse and human. Aim 2 is to map the locations of these active Ors and to sequence all OR-like sequences, including pseudogenes and ones functioning outside the nose, at each chromosomal location. Aim 3 is to characterize genomic sequence encompassing approximately 20 percent of the active OR genes to make paralogous and orthologous comparisons of gene structure and cluster organization. Aim 4 is to characterize the 5'UTRs and expression patterns of OR genes and to perform functional assays in mice to identify regulatory motifs and structural elements key for ligand-binding and neuron-targeting. Aim 5 is to develop technology to use cDNA arrays to examine expression profiles of 100's of OR genes at once. This partnership between the Axel group, with expertise in transgenics and olfactory biology, and the Trask and Hood groups, with experience in sequencing, genomic evolution, and cDNA-array technology, represents a unique opportunity to unite the worlds of large-scale genomics and functional biology. It is our belief that this sort of partnership in functional genomics represents a model for future collaborative efforts to characterize complex neurobiological systems and large gene families.